Study On Microfabric,Collapsibility And Structure Of Coastal Loess Around Bohai Sea

Coastal loess around Bohai Sea,another important loess deposit in China,is mainly distributed in the western of Liaodong Peninsula,Miaodao Islands and the Northern of Shandong Peninsula.The Loess along the coast is similar to the inland loess in terms of genetic mechanism and material composition.It also has unique engineering geological properties due to the material source and stress history different from the inland loess.Regarding the study of loess in the coastal zone,the present research results have a clear understanding of its distribution characteristics,forming age,genetic mechanism,stratigraphic lithology,paleomagnetic characteristics,grain size characteristics,mineral composition characteristics,chemical composition characteristics,etc.,but the understanding of its engineering geological characteristics is not clear enough.The relationship between microstructure characteristics,structural characteristics and collapsibility needs to be further studied.In this paper,Conventional geotechnical testing methods were accomplished to determine the physical properties.XRD,MIP and SEM technology were applied to obtain the mineral composition,pore distribution curves and microfabric images.Double oedometer experiments with different initial water contents were completed to get the collapsibility characteristics.Direct shear tests of undisturbed and remolded specimens with different initial water contents were made to gain the structural characteristics.The research results obtained include the following aspects:(1)the basic physical and mechanical properties of loess samples such as density,dry density,void ratio,natural moisture content,limiting moisture content,particle size distribution,mineral composition,pore distribution,compression characteristics and shear characteristics were obtained from 3 coastal zones of the Penglai County,the Beichangshan Island and the Beihuangcheng Island.(2)based on scanning electron microscope(SEM)images,we completed the qualitative analysis of the loess microstructure in 3 coastal zones along the north,Changshan island and Bei Huang Cheng island.The particle morphology and permutation characteristics,pore distribution characteristics and inter-particles cement characteristics of 3 samples were systematically described.The parameters of particles and pores,such as size,length,width,orientation,perimeter,area,fractal dimension and distribution entropy were extracted by PCAS software.Quantitative analysis of the microstructures of the specimens was completed by using these microstructural parameters.(3)Aiming at the problem that general SEM images can not take into account both horizon and resolution,the coastal loess SEM images magnified 50 times to 2000 times are obtained by using Quant 2000 scanning electron microscope,and a method of obtaining high-resolution SEM images in the same horizon is proposed,which solves the contradiction between horizon and resolution in the process of quantitative analysis.The results obtained by equivalent diameter method are close to those obtained by densimeter method and mercury intrusion porosimetry(MIP)test,which proves the reliability of the method.The results help to establishe one relationship between microstructural parameters and macro-physical parameters.(4)the collapsibility test of loess samples under different initial water cut conditions in coastal zone were completed.It was proved that the loess samples of 3 coastal zones such as BCS,BHC-1 and BHC-2 had moderate collapsibility,and PL specimens had slight collapsibility.Based on SEM images and MIP test results,the changes of microstructure parameters of loess samples in the coastal zone before and after collapsing were compared and analyzed.It is found that the number,volume and area of large and medium pores after collapsing decreases,and that of the small pore and micro-pore increases and remain unchanged respectively.The disorder degree of the macropore arrangement decreases significantly,while that of the small and medium pore arrangement remains basically unchanged.It is proved that the overhead pore represented by the medium pore is the main factor causing the coastal loess collapsibility.(5)The effects of void ratio,dry density,initial water content,pressure,particle composition,pore composition and intergranular bonding characteristics on collapsibility are discussed,with emphasis on the influence of particle composition on collapsibility.It is found that although fine silt and clay particles have different ways of affecting collapsibility of loess,the collapsibility of coastal loess shares the same tendency with the increase of silt and clay content,first increasing and then decreasing.(6)Direct shear tests of undisturbed and remolded samples of coastal loess with different initial water content have been completed.The soil-water characteristic curves of coastal loess have been measured,the shear behavior characteristics and the variation of shear strength parameters with pressure and water content have been analyzed,and two structural parameters based on sensitivity and comprehensive structural potential parameters have been constructed.It is found that the structure of coastal loess has a significant effect on its direct shear strength,and the influence of structure on shear strength parameters is different.The sensitivity of cohesion to structure is significantly higher than that of internal friction angle.The sensitivity of apparent cohesion and matrix suction to structure is significantly higher than that of apparent internal friction angle.With the increase of water content and the decrease of the matrix suction,the sensitivity coefficients of cohesion and apparent cohesion increase gradually,and the sensitivity coefficients of matrix suction internal friction angle decrease first and then stabilize,while the sensitivity coefficients of internal friction angle and apparent internal friction angle remain basically unchanged.(7)The effects of dry density,pressure,water content and microstructural characteristics on structural parameters are discussed.Under the same water content and pressure conditions,the structural parameters gradually decrease with the increase of dry density.With the increase of water content,the comprehensive structural potential parameters of coastal loess decrease,and the rate of reduction of the comprehensive structural potential parameters varies when the water contents were higher and lower than the plastic limit;similarly,there is a critical pressure value,and the rate of reduction of the comprehensive structural potential parameters varies in the two pressure intervals before and after the critical pressure value.Moreover,the critical pressure values are closely related to the stress history,pre-consolidation pressure and water content of the samples.The cementation of clay,calcium carbonate and soluble salts in Loess plays an important role in maintaining the structural stability of loess.(8)The relationship between collapsibility and structure of coastal loess is discussed.The structural property of coastal loess is the precondition for its collapsibility,and its stable development is the basis for its variable development.Variability parameters are closely related to the collapsibility coefficient.With the increase of water content,collapsibility and stability gradually decrease;with the increase of pressure,collapsibility first increases and then decreases,and variability gradually decreases.